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Determination of H 2 S and SO 2 in fumarolic gases is necessary to predict volcanic activities. This study evaluated three gas sampling methods (CdS precipitation, Ag 2 S precipitation, and I 2 oxidation) using artificial fumarolic gases with known compositions. H 2 S was initially fixed as CdS(s) or Ag 2 S(s) in the sulfide precipitations and S(0) in the I 2 oxidation. On the other hand, SO 2 was collected as dissolved species (SO 3 2 − and SO 4 2 − ). The Ag 2 S precipitation, which did not allow HCl determination in fumarolic gases by forming AgCl(s), significantly underestimated H 2 S and SO 2 due to the entrained loss of analytes as a result of water vaporization during H 2 O 2 treatment. The I 2 oxidation, which required duplicate sampling for complete analysis of H 2 S and SO 2 , tended to overestimate SO 2 probably due to difficulties in separating S(0) particles from solutions. The CdS precipitation produced more satisfactory H 2 S and SO 2 recoveries than the other methods. Additionally, the effect of delayed sample treatment was assessed by comparing H 2 S-to-SO 2 molar ratios resulting from immediate and one-week-delayed treatments after gas collection. The delayed treatment significantly lowered the ratios in the CdS precipitation likely due to the susceptibility of CdS(s) to oxidation. In the meanwhile, such an effect was minimal in I 2 oxidation. Similar results were also observed in the field evaluation of fumarolic gases at Mt. Hakone. Along with detailed sampling and treatment procedures, the causes for unsatisfactory H 2 S and SO 2 recoveries were discussed in the context of the underlying reaction schemes as well as potential experimental artifacts. Thus, this study helps to select and further modify appropriate gas sampling methods to meet specific needs. [ABSTRACT FROM AUTHOR]

Abstract: Chemical and isotopic compositions were measured for volcanic gases discharged from the recently erupted Heisei-shinzan dome of Unzen volcano. The volcanic gases are largely mixed with the air, whose content ranges from 60 to 90%. Chemical and isotopic compositions of the gases are corrected for the air mixing accounting not only for the major air components but also for contribution of the atmospheric moisture and CO2. The variation of the corrected composition revealed that these gases are derived from various sources including; 1) the air circulating through the lava domes, 2) the local meteoric water at various degrees of evaporation and vapor loss, 3) the magmatic CO2 likely originated from a deep source, and 4) the gases discharged from the cooling lava dome. The gases discharged from the cooling lava dome are characterized by the oxygen and hydrogen isotopic composition typical of the subduction-type volcanoes but with high Cl content and high Cl/S and F/S ratios. In contrast, discharges of CO2 with the magmatic δ 13C values are not correlated with discharge of heat or acid gas species, suggesting the ascent of CO2-rich fluids from a deep source, possibly through the volcanic conduit. [Copyright &y& Elsevier]

Abstract: During the dome-forming eruption in 1991 to 1995 at Mt. Unzen, Japan high temperature volcanic gases were sampled from fumaroles located at the western basement of the dacitic lava dome. Comparing the composition of volcanic gas, the volatile in the pre-eruptive melt of magma and the groundmass of lava, we propose that the volatile in magma was liberated through three different gas separation steps. The first gas separation happened at depth deeper than 3.9 km. The liberated gas traveled along the conduit then discharged as the observed fumarolic gas. The second gas separation took place beneath the lava dome. The liberated gas was discharged on the top of lava dome. The third gas separation occurred near surface in a so called “open system degassing” process. The large variation in the CO2/H2O ratio of fumarolic gas could be explained by the variation of the bubble fraction in the pre-eruptive melt of magma chamber before the gas separations. The estimated bubble fraction in the melt was less than 1.7 vol.%. Such a low fraction of bubbles in the magma chamber of Mt. Unzen would be one of the reasons why the eruption was non-explosive. The fumarolic gas in the later stage of eruption was enriched in HCl and HF due to the effect of open system degassing of magma, although the gas contained CO2 and H2S with high concentrations. The CO2 and H2S would be derived from a vapor phase of hydrothermal system developed around the conduit. [Copyright &y& Elsevier]